International piracy monitors say the number of overall attacks dropped slightly last year, and that Somali pirates remain the greatest threat.

The International Maritime Bureau said Thursday it recorded 439 pirate attacks in 2011, down from 445 a year before, breaking a four-year trend of increasing attacks worldwide.

It said Somali pirates were responsible for more than half the attacks. The report said that while there was an increase in incidents involving Somali pirates last year, the number of successful hijackings dropped by 43 percent.

The IMB credits the efforts of international naval forces patrolling the waters off Somalia, saying they are “critical” to combating piracy.

The report also identified the waters off Nigeria and neighboring Benin as particularly active spots for pirates. It said there is continued concern about the number of attacks off Nigeria that go unreported.